a) Field of the Invention
The present invention is directed to an imaging unit for ophthalmological devices by which a stripe pattern of known distance is imaged on the ocular fundus. In this way, with knowledge of the initial values serving as basis, it is possible to carry out absolute measurements of distances and objects at the fundus.
b) Description of the Related Art
The absolute size of an object can be calculated from an image of the fundus only when all of the geometric-optical data of the camera and particularly of the eye are known. While the axial length and radius of curvature of the cornea of the eye can be determined with relatively little expenditure using standardized devices, the measurement of the values of the refractive power of the rear surface of the cornea, the depth of the anterior chamber, the lens thickness, the refractive power of the lens and the ametropia, which are likewise necessary for an exact calculation, is only possible with a great expenditure and limited accuracy.
The nomogram developed by Littmann [1], from which the desired data can be determined approximately with knowledge of the ametropia and the curvature of the cornea, is of some help. This method, which is rather opaque and has not yet been automated, has not achieved popularity so that there is a need for a simple measuring method which is as automated as possible and which delivers clear measurement results. It should be possible to carry out this measuring method with an auxiliary device which is suitable, for example, for a fundus camera.
Ideally, the images of the fundus camera, the measured values of axial length and corneal curvature and the values of this auxiliary device should be prepared in such a way that the evaluation can be carried out automatically.
Solutions are known from the prior art in which a stripe pattern of known distance is imaged on the ocular fundus for absolute determination of distances and objects at the ocular fundus.
While the solutions of Rassow et al. [2] and Kennedy et al. [3] are still based on two-beam interference, Baumbach et al. [4] in 1989 proposed a laboratory construction in which laser interference fringes were projected on the fundus by a multiple-beam interferometer with Maxwellian imaging.
This solution for an eye examination instrument for measuring topographic data of the fundus of the eye is described in DE 36 21 983 A1. In this solution, high-contrast stripes are generated by a multiple-beam interferometer by Maxwellian projection and are imaged on the retina by a fundus camera. A laser light source whose laser light is reflected into the illumination beam path of the fundus camera is preferably used for generating the interference stripes. The Maxwellian projection guarantees that the imaging quality of the optical media of the eye being examined has no influence on the formation of stripes. The use of multiple interference causes the formation of stripes with a high contrast and the area of laser speckle is sharply restricted. The remaining speckle structure within the stripes is further reduced in contrast when images are recorded by the fundus camera using a strobe light, so that it no longer appears in a distracting manner when taking measurements. Reflections in the fundus recording can be prevented by coupling the laser light into the illumination beam path of the fundus camera.
Through simplified assumptions about the distance of the first principal plane of the eye lens from the corneal vertex and with knowledge of the axial length of the eye and of the curvature of the cornea, the stripe distance on the retina of the eye being examined could be calculated within a close approximation so that a scale was present on the object to be tested.
However, it was disadvantageous in the solutions mentioned above that it was necessary to take substantial actions in the fundus camera employed.
Literature:    [1] Littmann, H., “Zur Bestimmung der wahren Gröβe eines Objektes auf dem Hintergrund des lebenden Auges”, Klin. Mbl. Augenheilkunde 180: 286, 1982    [2] Rassow, B. and Wolf, D., “Die Messung der retinalen Sehschärfe mit dem Laserinterferenzgerät als klinishe Routinemethode”, Adv. Ophthalmol 34:116, 1977    [3] Kennedy, J. B., Schwartz, B., Takamoto, T., EU, J. K. T., “Interference Fringe Scale for Absolute Ocular Fundus Measurement”, Invest Ophthalm Vis Sci 24: 169, 1983    [4] Baumbach, P., Rassow, B., Wesemann, W., “Absolute Ocular Fundus Dimensions Measured by Multiple-Beam Interference Fringes”, Invest Ophthalm Vis Sci 30: 2314, 1989